Tag Archives: eukarya

Nick Lane Lecture

Enjoy this lecture, as it echoes many of the points i have made on this blog:

Coyne vs. Shapiro

Jim Shapiro has been outlining his views on evolution over at the Huffington Post, including a posting entitled, What Is the Key to a Realistic Theory of Evolution?

Not surprisingly, Jerry Coyne does not like it and weighs in with a posting entitled, A colleague wrongfully disses modern evolutionary theory.

Let me focus on a key point of their disagreement.

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Scientific discovery, not tautology

I previously showed that the scientific discovery of a complex LECA was not a tautology.  DrREC replied:

Mike Gene, do you recognize the difference between FIRST and LAST?

L as in LUCA or LECA is LAST-The Last Eukaryotic Common Ancestor, the most RECENT (not oldest) organism from which all organisms/all Eukaryotic organisms (respectively) living on Earth descend.

Not the First! Do you understand LECA isn’t the first Eukaryote?

This is a very strange line of questioning given that nowhere did I argue that the last eukaryotic common ancestor was the first Eukaryote.  Nor do I think so.  Thus, DrREC’s first objection amounts to shadow boxing with the straw man he invented.

But it gets worse.

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Complex LECA is no tautology

Someone with the moniker DrREC replied to my posting about the complexity of the last eukaryotic ancestor as follows:

This is almost a tautology. The last Eukaryotic common ancestor had the defining features of a Eukaryote….which happen to be more complex than prokaryotic life.

There is no tautology at work here.  Not even close.  We can appreciate this by simply recognizing that scientists could very well have discovered that LECA was remarkably simple.  For example, it could have been a cell with a nucleus, but lacking protein-coding introns, mitochondria, golgi bodies, ubiquitin, and flagella.  And its nuclear pore complex, cytoskeleton, and endomembranous system could have been rather simple.  But as it turned out, LECA had a level of complexity that rivals modern day cells.

Of course, we don’t need to be hypothetical about this.  Back in the 1980s, biologists expected LECA to have been rather simple.  Consider the simplest of eukaryotic cells – microsporidia.

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Koonin and LECA

Below the fold you will find some excerpts from Eugene Koonin’s article, The origin and early evolution of eukaryotes in the light of phylogenomics.

 

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Oh my. LECA was really complex.

Earlier I showed you that the last eukaryotic common ancestor (LECA) was quite modern-like in terms of its nuclear pore complex, mechanisms of transport through this complex, and the entire endomembranous system.  Yet the modern-like features do not stop there.

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LECA Front-loaded Metazoa?

In the previous posting, we saw that the last common ancestor of all eukaryotic organisms was quite modern-like in terms of its complexity.  Doubt me?  Well, here is Figure 3 – Major transitions in evolution of the endomembrane system – from Evolution of the eukaryotic membrane-trafficking system: origin, tempo and mode).  Have a look:

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Ancestral Eukaryote with modern-like complexity

Recall that the eukaryotic cell plan is needlessly complex.

Recall the evidence suggests this needless complexity was essential for the emergence of metazoan-type existence.

And it looks like the key feature that facilitated the emergence of metazoan-type complexity is the nucleus (see here and here).

Of course, the nucleus, even without the chromosomes within, is a very complex and sophisticated structure.  Yet just how old is this complexity?

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Eukaryotes and Prokaryotes

I’ve long found it fascinating that every living thing on this planet can be cleanly split into two categories – prokaryotes and eukaryotes.  The prokaryotes consist of all the bacteria while the eukaryotes include animals, plants, fungi, and various protozoa.  The core life processes of the two cells are much the same, being built around the triad of proteins, RNA, and DNA, relying on the ribosome to build the proteins that synthesize everything else, including RNA and DNA, using ATP as the primary energy currency, and using lipid bilayer membranes to compartmentalize.  So what makes the two cell plans so different?

Below is a nice figure that helps you answer this question.

 

As you can see, there are two primary differences: size and level of compartmentalization.  Typical eukaryotic cells are much larger than bacteria and show a much more extensive level of compartmentalization given the numerous membrane-bound organelles and membranous folds.

Yet a question to ponder is why there are two cell types and only two cell types?  The non-telic perspective would explain this (away?) as simply an artifact of a contingent past.  There is no reason to ponder the question “why?”  It just happened that way.  But the telic perspective allows us to think of these two cell plans at a level that runs deeper.

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Nudging Mitochondria Along

It the previous posting, we saw there was good reason to think mitochondria were a necessary prerequisite for the evolutionary emergence of metazoan-type complexity.  Again, as Lane and Martin clearly point out:

Our considerations reveal why the exploration of protein sequence space en route to eukaryotic complexity required mitochondria. Without mitochondria, prokaryotes—even giant polyploids—cannot pay the energetic price of complexity; the lack of true intermediates in the prokaryote-to-eukaryote transition has a bioenergetic cause.

So we can see that natural selection, functioning as a designer-mimic, is, like other designers, constrained (and thus guided) by the materials used to express the design.  Just as there is no reason to think natural selection could craft something as complex and sophisticated as the prokaryotic cell without proteins, natural selection apparently cannot craft something as complex as a mouse or squid without the eukaryotic cell plan.  That’s why cells had to be first re-tooled through an endosymbiotic relation.

But why haven’t bacteria, after billions of years, ever been able to discover a method of evolving something mitochondrial-like without relying on endosymbiosis?   At first, it might seem to be simply an issue of scale, as the typical mitochondrion is roughly the same size as the typical bacteria.  But there are bacteria that are as large as some eukaryotic cells and it looks like they try to mimic mitochondria, but never quite make it.  One such bacterium is Thiomargarita.  

Lane and Martin discuss this bacterium and Myers summarizes their argument as follows:

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